konzel@utecfc.UUCP (Charles Konzelman) (09/13/85)
About a year and a half ago I bought a Sony SL5200 Beta HiFi VCR with the intention of using it solely as an audio recorder. I was unable to locate a dealer that could demonstrate it properly, so I purchased it on the basis of its excellent specifications and the favorable reviews in the press. However, on playback the unit seems to have some audio problems: 1) Levels of hum and hiss seem inordinately high, and I find this rather obstrusive in quiet passages. 2) A whining noise is heard at the beginning of each Beta HiFi recording, but this gradually decays. 3) Sometimes the Beta HiFi light drops out for a second or so, with a corresponding loss of hi-fi, but then comes back on again. 4) There are random, short duration, low level, buzzing sounds (could these be tape dropouts?) which are especially apparent (and annoying) during quiet passages. 5) Worst of all, a distortion problem which seems level and frequency dependent. This is really apparent with solo recordings (for example, a female opera singer or a trumpet) where there is no other music to mask the problem. I would describe the distortion as a harsh, jagged kind of sound which accompanies the music. This occurs at levels below the red LED (0dB). I recorded some sine wave test tones at different levels and frequencies. The performance seems OK at low frequencies, but at high frequencies the same harsh, jagged sound emerges, somehow superimposed over the test tone. This occurs at levels below the red LED, and higher levels aggravate the problem. The Sony Service Centre tells me (after numerous visits) that there is nothing wrong with my machine and that all SL5200's behave the same way. The special "HiFi" tapes improve the performance, but I really can't believe the unit is meeting the excellent specifications Sony touts so highly. Has anybody out there had any experiences with these HiFi VCR's (in particular the Sony SL5200)? Do all of these machines behave in a similar fashion, or is there something wrong with my unit? NOTE: On some SL5200's the LEDs go from -20dB to +8dB, but on mine they go from -30dB to 0dB. What is the significance of this? I would appreciate ANY feedback on the subject, as this problem really has me frustrated. Very sincerely, Charles Konzelman University of Toronto Toronto, Ontario Canada
raan@hp-pcd.UUCP (raan) (09/26/85)
I am not familiar with your particular unit, but I own a Sony SL-2700 Beta HI-FI. >> 1) Levels of hum and hiss seem inordinately high, and I find this rather >> obstrusive in quiet passages. My unit has the ability to play either the HI-FI or the linear sound tracks, or both. Are you sure you do not have the linear track on. This is much noisier than the HI-FI sound and if both were on would show up as noise with out affecting the frequency response (which is the other major difference between the two). >> 2) A whining noise is heard at the beginning of each Beta HiFi recording, but >> this gradually decays. I have not experienced anything like this. Does your unit have indexing (if it does, it may be out of adjustment and affecting the sound, since it records a mark on the tape at the beginning of the record session). To test this, try positioning the tape in the middle of a section of already recorded tape and then go from play to record without pressing stop. (Press Play, Pause, Rec, and then Pause to start recording -- assuming yours works like mine.) This will start the recording without putting an index mark on the tape. See if this still generates the noise. >> 3) Sometimes the Beta HiFi light drops out for a second or so, with a >> corresponding loss of hi-fi, but then comes back on again. This is normal behaviour for the system when there are dropouts on the tape. If this happens with several different new tapes, then the dropout sensor in the unit may be out of adjustment. A dropout will also manifest itself as a horizantal glitch in the picture, usually moving from top to bottom of the screen. If all the tapes you try were from the same lot, then you may still have a batch of bad tapes. >> 4) There are random, short duration, low level, buzzing sounds (could these >> be tape dropouts?) which are especially apparent (and annoying) during quiet >> passages. Try playing with the tracking control. The HI-FI sound is very sensitive to tracking errors, especially at Beta III. The problem will show up as clicks and pops or low frequency (30Hz to be exact) sharp toned buzzing. This is also related to the next item. >> 5) Worst of all, a distortion problem which seems level and frequency >> dependent. This is really apparent with solo recordings (for example, a >> female opera singer or a trumpet) where there is no other music to mask >> the problem. I would describe the distortion as a harsh, jagged kind of >> sound which accompanies the music. This occurs at levels below the red >> LED (0dB). I recorded some sine wave test tones at different levels and >> frequencies. The performance seems OK at low frequencies, but at high >> frequencies the same harsh, jagged sound emerges, somehow superimposed >> over the test tone. Because of the way the signal is recorded on the tape (in 1/30 second slices) the playback signal contains a 30 Hz switching transient. The system has circuitry to remove this noise and normally does very good job of it. However, (for reasons I don't understand) it fails miserably on high tones without much background. The worst case of this being test tones. This happens on both Beta and VHS HI-FI and has been reported in Video Review. I have confirmed it on all of my machines. There does not seem to be anything that can be done about it. >> The Sony Service Centre tells me (after numerous visits) that there is >> nothing wrong with my machine and that all SL5200's behave the same way. The >> special "HiFi" tapes improve the performance, but I really can't believe the >> unit is meeting the excellent specifications Sony touts so highly. Other than the last problem mentioned, I have been reasonably happy with my machines. However, I am somewhat upset about the claims made for HI-FI sound (on all machines, not just Sony) in light of this problem. I feel that the advertising is misleading. --- Raan Young (hp-pcd!raan) ~~~~~~~~~~~~~~~ uoregon ---->\ orstcs ----> \ hplabs ----> \ ----> >--!hp-pcd!raan harpo ----> / microsoft ----> / tektronix ---->/ ~~~~~~~~~~~~~~~
die@hydra.UUCP (Dave Emery) (10/05/85)
> >> 5) Worst of all, a distortion problem which seems level and frequency > >> dependent. This is really apparent with solo recordings (for example, a > >> female opera singer or a trumpet) where there is no other music to mask > >> the problem. I would describe the distortion as a harsh, jagged kind of > >> sound which accompanies the music. This occurs at levels below the red > >> LED (0dB). I recorded some sine wave test tones at different levels and > >> frequencies. The performance seems OK at low frequencies, but at high > >> frequencies the same harsh, jagged sound emerges, somehow superimposed > >> over the test tone. > > Because of the way the signal is recorded on the tape (in 1/30 second > slices) the playback signal contains a 30 Hz switching transient. The > system has circuitry to remove this noise and normally does very good > job of it. However, (for reasons I don't understand) it fails miserably > on high tones without much background. The worst case of this being test > tones. This happens on both Beta and VHS HI-FI and has been reported > in Video Review. I have confirmed it on all of my machines. There > does not seem to be anything that can be done about it. > --- Raan Young (hp-pcd!raan) I believe that there is quite simple explanation of this phenomonon. It has to do with the effect of a slight time delay error in the signal coming from one spinning head versus the signal coming from the other. Assuming a clever and distortionless circuit for handling the switch between heads, there still exists a form of distortion that should be worst on pure high frequency notes. That distortion is phase modulation of the high frequency signal at the head switch rate due to slight errors in the angular position of one head versus it's opposite number 180 degrees across the head drum. This can cause one head to be reading FM audio delayed by a few hundred nanoseconds to a couple of microseconds or so (1 us corrosponds to 39 seconds of arc in head alignment for a head drum spinning at 1800 rpm). The effect of this difference in delay is to cause a phase shift in the audio demodulated from the fm carrier every time the electronics switches from one head to the other. Or in other words to phase modulate the high frequency notes with a 30 hz square wave. At 10 khz a signal has a period of 100 us and a jitter of as little as 3 microseconds would cause a phase modulation of 10 degrees. A phase modulation of 10 degrees results in first order sidebands about 21 (approx) db below the tone, this corrosponds to a 'IM' distortion of about 10 percent (Pardon my simple estimates, I haven't bothered with more terms of the series than the first couple). I am not an expert on phychoaccoustics but I suspect that a tone this impure (with sidebands stretching out from + and - 30 hz) won't sound as sweet as a nice pure 10 khz tone with little or no sideband energy near it. Obviously the distortion is much less at lower frequencies. It may very well be that this distortion is much less audible when AM modulation (by instrument and musician) generates the dominent sideband energy, thus only passages with pure high frequency notes will sound bad. It is also possible that the head switching electronics also introduce a transient due to balence errors of various sorts between the channels. I do not know whether this is worse in magnitude than the phase modulation. The time delay could be corrected by use of ccd or other types of analog delay elements to compensate for the mechanical errors in the heads (the tv video sync pulses make a nice reference for allowing such a circuit to correct dynamically). This kind of time base correction is actually applied to video coming out of broadcast vtrs, and it would substantially eliminate the error due to this cause but would probably be expensive. David I. Emery Charles River Data Systems 617-626-1102 983 Concord St., Framingham, MA 01701. uucp: decvax!frog!die
die@hydra.UUCP (Dave Emery) (10/18/85)
> > >> 5) Worst of all, a distortion problem which seems level and frequency > > >> dependent. This is really apparent with solo recordings (for example, a > > >> female opera singer or a trumpet) where there is no other music to mask > > >> the problem. I would describe the distortion as a harsh, jagged kind of > > >> sound which accompanies the music. This occurs at levels below the red > > >> LED (0dB). I recorded some sine wave test tones at different levels and > > >> frequencies. The performance seems OK at low frequencies, but at high > > >> frequencies the same harsh, jagged sound emerges, somehow superimposed > > >> over the test tone. > > I believe that there is quite simple explanation of this > phenomonon. It has to do with the effect of a slight time delay > error in the signal coming from one spinning head versus the signal coming > from the other. Rereading my own posting by happenstance, another thought occurs to me. The bandwidth of the filters used to split out the specific channel of FM sound (one of four frequencies) may not be wide enough to reproduce heavily modulated signals modulated by high frequency tones especially if a fair degree of preemphasis is used. I do not remember how much preemphasis is actually used with Beta Hifi, but I suspect it increases the deviation of a 10 khz signal by several db over a 1 khz reference. If this is true than a maximum amplitude 10 khz signal (recorded at 0 VU ref) might well modulate the subcarrier to such a swing as to exceed the bandwidth of the filters used to separate it from it's companion signals nearby. And with a modulation index of >4 or 5 (guessing) the sideband energy should spread out quite some additional distance. All of this would cause a very level sensitive distortion with a very steep onset at a recording level threshold near to but possibly not at maximum. The resultant distortion could be very severe if it caused gross phase errors in the sideband components (and particularly falls through the origen as in the near FM threshold click phenomenon). It's quite possible that the choice of FM deviation (recording level) was made to maximize the signal to noise ratio with the full knowlage that the resultant system was incapable of recording full amplitude high frequency signals. MOST music does not contain passages with very high amplitude high frequency notes in the absence of enough energy at lower frequencies to cause considerable gain reduction in the compander (dBx) so perhaps the desire for extreme SNR led to a deliberate overmodulation. David I. Emery Charles River Data Systems 617-626-1102 983 Concord St., Framingham, MA 01701. uucp: decvax!frog!die